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Pressure is building up under the Crater Lake on Mt Ruapehu, increasing the likelihood of an eruption in the coming weeks, GNS Science said.

Recent measurements taken on the volcano over the last few weeks indicated that eruptions were more likely "over the next weeks to months".

A sudden release of pressure built up under the lake may lead to an eruption.

The Department of Conservation has issued a warning for trampers and climbers to avoid the Summit Hazard Zone, a two kilometre area from the centre of the Crater Lake.

“We recommend that guiding companies do not take people into the zone. Any other climbers, trampers and walkers should not enter the zone” DOC Ruapehu area manager Jonathon Maxwell said.

People in the area should be aware of an increased possibility of lahars on the tracks and especially when approaching the Whangaehu, Wahianoa, Mangaturuturu and Whakapapaiti streams.

“They should be conscious of potential lahar noise from upstream, and make their way across these streams quickly,” DOC warned.

Mt Ruapehu, which last erupted in 2007, usually does not give any immediate warning that it is going to erupt, GNS Science vulcanologist Steven Sherburn said.

Pressure build-up was thought to have caused the 2007 eruption and a smaller eruption in 2006.

The Aviation Colour Code has increased from green to yellow as a warning for those flying over the region, but the Volcanic Alert Level remained at level 1.

GNS Science believed the temperature a few hundred metres beneath the crater was about 800 degrees Celsius but the lake itself was only 20degC.

"This suggests the vent is partly blocked which may be leading to a pressure build-up beneath Crater Lake.''

Small earthquakes have been occurring about five kilometres beneath the summit of Ruapehu since late October, but it was not known whether they were related to the increased temperature below the crater, GNS vulcanologist Brad Scott said.

"We perceive them to be completely unrelated at this stage," he said.

However, the quakes had the potential to build up processes and push magma further to the surface, which would further increase the likelihood of an eruption, he said.